The design of high-voltage-system components, such as dry transformers, uses insulation means. For example, when there is little separation between two components operated at different voltage levels (for example, between an upper- and lower-voltage winding of a transformer), electrical breakdown can result. In order to prevent this, insulation barriers can be used which increase the flashover length and thus provide increased reliability of operation of the component. In the case of nested lower- and upper-voltage windings in a high-voltage dry transformer (for example, for a voltage level between 6 kV and 30 kV), this type of barrier is designed as a thin-walled insulating hollow cylinder arranged within the radial region that separates the lower-voltage winding, which can be located radially inwards, from the upper-voltage winding, which can be located radially outwards.
The cylinder can be composed, for example, of a thin-walled multilayer laminate. Laminate is the term used for a material or a product which is composed of two or more layers where large surfaces have been bonded to one another by adhesion or by other means. These layers can be composed of identical or different materials. A laminate suitable for the abovementioned insulation purpose can have, for example, the following layer structure, where the polyester foil can determine electrical strength:                8 μm of hardened polyester imide resin or epoxy resin;        50 μm of polyester non-woven;        350 μm of polyester foil;        50 μm of polyester non-woven;        350 μm of polyester foil;        50 μm of polyester non-woven;        8 μm of hardened polyester imide resin or epoxy resin.        
It is also possible to use a polyester foil adhesive-bonded from multiple plies (for example, up to 4 plies), the layer thickness in each case being 350 μm.
This type of laminate, resembling a strip, is produced in roll form by means of appropriate lamination equipment. In order to convert this to the desired hollow cylinder shape of an insulation barrier for a high-voltage transformer, a strip section of desired length and width is manually bent into a hollow cylinder shape and fixed at an overlap by a heat-resistant adhesive tape. It can be desirable that the entire structure has a heat resistance in temperature class F, i.e., 155° C., for application in dry transformers in the high-voltage sector.
A disadvantage here is that the first-mentioned laminate structure with its overall thickness of about 1 mm can have restricted stability which is not suitable for the production of hollow insulation cylinders for relatively large transformers, for example, for a rated power of 5 MW and above. Addition of further layers with a resultant higher overall thickness involves increased cost and use of material, and can be disadvantageous. Another disadvantage is that this type of insulation barrier can contribute to a fire load of a transformer.
Although the four-ply polyester foil described as an alternative has slightly higher mechanical stability, because of a lack of a coating on the external sides of the foil composite it may only be suitable for temperature class B, i.e., 130° C.